You can find them year round at some stores like outdoor stores and Old Navy. They become more popular in stores around early March when the Spring line comes into the stores.
It really depends on the store. Regular shoe stores and some large discount stores will often carry at least a small collection of flip-flops year round. The largest collection of flip-flops usually begins to come out in early spring as people begin thinking about summer weather and vacations.
I work in a large retail store, in the shoe department, and our flip-flops came in late in March. We stored them in the back for a week or so and then brought them out the first of April. Check the stores again!
I usually see flip-flops come out in between the February and March months, as people are starting to get prepared for the upcomming Spring and Summer weather. You could always shop online as well.
